Help with writing textfiles

prometheus.scm

@@ -27,6 +27,7 @@
   #:export (make-metrics-registry
             metrics-registry-fetch-metric
             write-metrics
+            write-textfile
 
             make-counter-metric
             make-gauge-metric
@@ -409,3 +410,24 @@ so that it can receive and store the metric values."
            value))
         (metric-values metric))))
    (metrics-registry-metrics-hash registry)))
+
+(define (write-textfile registry filename)
+  "Write all metrics from the given @var{registry} to @var{filename}
+in the standard text based exposition format.
+
+For the node exporter to read the file, the @var{filename} must end
+with .prom.
+
+This procedure takes care of atomically replacing the file."
+  (let* ((template (string-append filename ".XXXXXX"))
+         (out      (mkstemp! template)))
+    (with-throw-handler #t
+      (lambda ()
+        (chmod out(logand #o666 (lognot (umask))))
+        (write-metrics registry out)
+        (close out)
+        (rename-file template filename)
+        #t)
+      (lambda (key . args)
+        (false-if-exception (delete-file template))))))
+